1. FIELD OF THE INVENTION
The present invention relates to facilitating the maintenance of nuclear power systems and particularly to minimizing the exposure to radioactivity of service personnel working within the steam generator of a nuclear power system during the performance of routine periodic maintenance thereon. More specifically, this invention is directed to a radiation shielding system employing shielding modules or tiles which may be rapidly and easily installed in steam generators.
2. DESCRIPTION OF THE PRIOR ART
While not limited thereto in its utility, the present inventive system has been designed for use in, and has special utility for, the servicing of the steam generators of nuclear power systems, particularly such systems which employ pressurized water type reactors. Such steam generators comprise a pressure vessel having a lower plenum area which is a hemisphere of five to seven feet radius divided into two halves. A coolant, which has been heated in the reactor, is delivered to one of the plenum halves and is then circulated, via a bundle of tubes, which may contain from 3,000 to 11,000 tubes supported by a horizontal tube sheet, through the steam generator vessel. The coolant is subsequently discharged from the other plenum half and returned to the reactor. During passage through the steam generator tube bundle, heat from the reactor coolant is transferred to water under pressure which subsequently flashes to steam for driving a turbine.
Nuclear Regulatory Commission rules require periodic inspection of the steam generators of nuclear power systems. To this end, the steam generator vessel is provided with access openings, known in the art as "manways", in the lower plenum area. During normal operation these access openings are sealed by means of covers. In order to perform inspections, after the vessel has cooled and the "primary loop" has been drained of reactor coolant, the manway covers are removed.
Once access to the interior of the steam generator pressure vessel is possible, a number of different procedures may have to be performed within the vessel. However, since the interior of the steam generator vessel is classified as a highly radioactive environment, maintenance personnel may work in the lower plenum area for only short periods of time. The types of procedures which may have to be performed from within the lower plenum area of a stem generator vessel include non-destructive testing, steam generator tube pulling, steam generator tube plugging, installation of sleeves in steam generator tubes and the installation of nozzle dams to prevent backflow of coolant from the reactor pressure vessel should it be necessary to flood the reactor in order to perform separate operations thereon.
The non-destructive testing will typically comprise ultrasonic and/or eddy current examination of the interior of a preselected percentage of the steam generator tubes. For example, 3% of the tubes will be tested to determine if there is any reduction in effective wall thickness, i.e., cracks, pits, or corrosion, of greater than 20% or any growth in effective wall thickness, i.e., scaling or other deposits, of more than 20%. If a preselected number of the tested tubes are found to exceed the set limits for increased or reduced wall thickness, an additional percent of the tubes will be tested.
Steps must be taken to minimize the radiation exposure of service personnel who are working under the tube sheet in the lower plenum area. Prior attempts to provide the requisite radiation shielding in the lower plenum area below the tube sheet have largely been limited to hanging lead blankets under the open tube ends and tube sheet on scaffold-like racks. This has not proven to be a satisfactory procedure since the racks and blankets take a long time to install, and the installers are subjected to radiation during the installation.
The recent development of a new "Rapid Installation Tube Gripper" invention has provided a fastening device which made the modular radiation shielding system of the present invention practical. The gripper was invented by Glenn E. Schukei, one of the joint inventors of the instant invention, and Robert J. Schukei, and is the subject of U.S. patent application Ser. No. 686,114, filed Dec. 24, 1984.
The gripper device can be rapidly attached to the inside of a steam generator tube to secure a modular lead shielding tile. The device uses hard balls which are held in position by a sleeve so that the balls can be wedged between a tapered shaft and the wall of the tube when an attempt is made to withdraw the tapered shaft. The harder the shaft is pulled, the tighter the balls are wedged, thus insuring that they cannot be inadvertently withdrawn. To release the device, the tapered shaft is inserted further into the tube than the sleeve, so that the balls are relocated relative to the tapered surface. The released position of the balls and tapered shaft is maintained by the sleeve until the device is removed from the tube.
A single worker is permitted to stay in the lower plenum without shielding for only two or three minutes, during which time he may be exposed to his three month radiation dose limit. From this it can be seen that any system that can save even a few seconds or can shield any significant radiation is quite valuable.